Dual roles of the inorganic aqueous phase on secondary organic aerosol growth from benzene and phenol

Abstract

Benzene, emitted from automobile exhaust and biomass burning, is ubiquitous in ambient air. Benzene is a precursor hydrocarbon (HC) that forms secondary organic aerosol (SOA), but its SOA formation mechanism is not well studied. To accurately predict the formation of benzene SOA, it is important to understand the gas mechanisms of phenol, which is one of the major products formed from the atmospheric oxidation of benzene. Laboratory data presented herein highlight the impact of the aqueous phase on SOA generated through benzene and phenol oxidation. The roles of the aqueous phase consist of (1) suppression of the aging of hydrocarbon and (2) conventional acid-catalyzed reactions in the inorganic phase. To explain this unusual effect, it is hypothesized that a persistent phenoxy radical (PPR) effectively forms via a heterogeneous reaction of phenol and phenol-related products in the presence of wet inorganic aerosol. These PPR species are capable of catalytically consuming ozone during an NOx cycle and negatively influencing SOA growth. In this study, explicit gas mechanisms were derived to produce the oxygenated products from the atmospheric oxidation of phenol or benzene. Gas mechanisms include the existing Master Chemical Mechanism (MCM v3.3.1), the reaction path for peroxy radical adducts originating from the addition of an OH radical to phenols forming low-volatility products (e.g., multi-hydroxy aromatics), and the mechanisms to form heterogeneous production of PPR. The simulated gas products were classified into volatility- and reactivity-based lumped species and incorporated into the Unified Partitioning Aerosol Reaction (UNIPAR) model that predicts SOA formation via multiphase reactions of phenol or benzene. The predictability of the UNIPAR model was examined using chamber data, which were generated for the photooxidation of phenol or benzene under controlled experimental conditions (NOx levels, humidity, and inorganic seed types). The SOA formation from both phenol and benzene still increased in the presence of wet inorganic seed because of the oligomerization of reactive organic species in the aqueous phase. However, model simulations show a significant suppression of ozone, the oxidation of phenol or benzene, and SOA growth compared with those without PPR mechanisms. The production of PPR is accelerated in the presence of acidic aerosol and this weakens SOA growth. In benzene oxidation, up to 53 % of the oxidation pathway is connected to phenol formation in the reported gas mechanism. Thus, the contribution of PPR to gas mechanisms is less than that of phenol. Overall, SOA growth in phenol or benzene is negatively related to NOx levels in the high-NOx region (HC ppbC / NOx ppb < 5). However, the simulation indicates that the significance of PPR rises with decreasing NOx levels. Hence, the influence of NOx levels on SOA formation from phenol or benzene is complex under varying temperature and seed type conditions. Adding the comprehensive reaction of phenolic compounds will improve the prediction of SOA formation from aromatic HCs due to the missing mechanisms in the current air quality model.